This invention relates to a continuous forming method to produce various long stringers, stiffeners, panels and frames made from composite materials where the forming apparatus is considerably smaller than the length of the produced parts.
Current methods available to manufacture long composite structures such as stringer, stiffeners, panels and frames of various cross sections are autoclave or pultrusion processing. The autoclave process involves having a male or female tool of the desired shape and length. After the desired lay-up is completed in the tool using unidirectional tape, woven cloth or other material, it is vacuum bagged and put into an autoclave oven where it is heated to a melting temperature of the resin, then pressure is applied to the whole part within the autoclave and this pressure is maintained until the part cools off sufficiently to allow the part to maintain the shape of the tool without any pressure. When this stage is reached, the pressure is turned off and the part is removed from the autoclave. While this method can produce quite suitable parts, it has a number of disadvantages.
(a) The tool has to be as long as the part being produced so if a 50' long part is needed, the tool has to be at least 50' long. A tool of this size can be very expensive.
(b) Since the whole part is heated and under pressure in the autoclave, the equipment has to be large enough to accommodate the whole tool. Again, in the case of a large part, it can be very expensive to operate, heat and cool a large autoclave.
(c) Heating and cooling the large autoclave is time consuming so production rate is slow resulting in expensive fabricating costs.
Producing long parts by pultrusion has its problems and limitations. In this concept a metal pultrusion die is used having an opening of the same configuration as the desired cross section of the part to be produced. The material is in the form of resin impregnated fiber tows. This material is first heated to the resin's melting temperature, then it is pulled continuously through the opening of the pultrusion die. During this process the part is being cooled off and as it exits the pultrusion die it has solidified to the desired shape. While this process is very economical due to the pultrusion speed, it has very serious limitations.
(a) For maximum strength designers like to have a large percentage of fiber content (60-70%) since the fibers are the ones that carry most of the load. For this same reason they like to have the large percentage of fibers aligned to the direction of the main stress the part is subjected to during use. This, of course, means that most of the fibers are angular or perpendicular to the length of the part. This kind of fiber orientation is very difficult or impossible to obtain with pultrusion, therefore, pultruded parts have most of the fibers running longitudinally within the part which severely limits the part's strength in more than the one direction.
(b) To achieve maximum strength a composite part has to be well consolidated, that is, to have a minimum amount of voids and porosity. This can be achieved by applying a considerable amount of pressure during forming when the resin is still in the melted stage. This kind of pressure cannot be applied to the part during pultrusion. During the process the material is being pulled through the pultrusion die and the material is contacting and slipping by the inner wall of the die opening. This results in friction between the two surfaces. Large pressure on the material (that is needed for good consolidation) results in larger friction, making it harder to pull the composite material through the pultrusion die resulting in fiber breakage and stopping the pultrusion process. Pultruders try to overcome this problem by putting more fibers in the longitudinal direction and have a larger percentage of resin to accommodate slipping between die surface and the pultruding material. Also, this problem with friction limits the possibility of placing fibers in an angular or perpendicular direction of the part during pultrusion.